Fluororesins are widely used in the form of a coating composition which is applied to a substrate for a product (e.g. molds for bread, rice kettles) requiring properties such as corrosion resistance, non-stickiness, and heat resistance, so that a fluororesin layer is formed. Fluororesins, however, have poor adhesion to substrates made of, for example, metal or ceramic because of their non-non-stickiness. Hence, before the application, a substrate is coated with a primer having compatibility with both fluororesins and the substrate.
For uses requiring corrosion resistance, generally, a fluororesin layer needs to be thick. To produce a thick layer, coating has to be repeated which includes applying a coating powder containing a fluororesin and baking the material at temperatures not lower than the melting point of the fluororesin. The primer used is required to have sufficient heat-resistant adhesion to bear long-time high-temperature baking and maintain adhesion to components such as a substrate.
Primers with excellent heat-resistant adhesion having been widely used are chromium (III) phosphate primers which have excellent resistance to long-time high-temperature baking. However, the concerns about the environmental problems have increased, and thus have led to a long-time strong demand for development of chromium-free primers that do not contain hexavalent chromium but have heat-resistant adhesion as high as chromium (III) phosphate primers.
The chromium-free primers having been studied up until now are combinations of fluororesins and various binder resins. For binder resins, use of polyphenylene sulfide [PPS] has been suggested in terms of heat resistance. PPS, however, has a disadvantage of poor compatibility with fluororesins, exhibiting insufficient adhesion to fluororesin layers.
To overcome the disadvantage, technologies for improving adhesion to fluororesin layers have been developed. Patent Literature 1, for example, discloses a coating powder which contains a macromolecule compound (A) having an amide group and/or imide group, an anti-oxidizing material (B) and a fluororesin (C), wherein the macromolecule compound (A) has an average particle size of smaller than 50 μm.
Patent Literature 2 discloses a composition usable as a multilayer coated primer, which comprises a plurality of multicomponent particles, one component of each of the particles being melt-fabricable fluoropolymer and another component of each of the particles being a high temperature resistant non-dispersed polymer binder. The composition of Patent Literature 2 is in the form of powder which is free-flowing and not subject to segregation of components during shipping or processing.
Although not for use as a primer, Patent Literature 3 discloses a method for producing a heat-meltable fluororesin composite composition which includes a process (I) of obtaining a heat-meltable fluororesin powdery mixture composition by mixing heat-meltable fluororesin fine powder and a layered compound; and a process (II) of melting and mixing the powdery mixture composition under shear stress by means of a melt-mixing extruder.